Bad language when used therapeutically by the royal family is not that awful, the British Board of Film Classification decided today as it abruptly downgraded its classification for a movie depicting George VI's battle with his stammer.

The King's Speech, a funny and moving film that tells the story of the monarch's vocal difficulties, had been given a 15 rating, much to the annoyance of its director.

The film has its UK premiere at the London Film Festival tonight and its actors and directors spoke of their bafflement at the rating, given because of a comic scene in which the then Duke of York is encouraged to swear as part of his speech therapy.

The film's director, Tom Hooper, best known for Longford on TV and movie The Damned United, said he could not understand the logic.

"My head is in my hands about it," he told a press conference after the first British screening. "I go to see Salt, where a tube is forced down Angelina Jolie's throat and then water is poured down her throat to simulate drowning, that's not a problem. The last Bond film, when Daniel Craig's bollocks are smashed in through a chair with no bottom and then there's another torture scene. These are scenes that are still in my head which I don't want in my head, they're troubling me and I'm my age."

Stung by the comments, the BBFC later announced that its classification decision had been given "careful consideration" by its president and director. The board took the view it would be suitable for the less restrictive 12A rating. The BBFC added: "The public would understand why the board has reached this decision."

Colin Firth, who plays the reluctant king, said the context of the words was everything – that they were not being used in a sexual way or to insult or describe anyone. "It would be very interesting to know who the people are who would complain about that stuff ahead of violence."

Firth said he had done a lot of research into stammering for the role. "This the third time I've played somebody with a stammer and what's interesting is you don't just pull your stammer from a drawer and that was an education for me because I thought perhaps you could.

"What you're really playing is not stammering, because that's what the person is going through."

The King's Speech, part-funded by the now scrapped UK Film Council, is based on real events that have, necessarily, been imagined. Hooper said: "Everyone knows that the Royal Family's ability to control the flow of information from the palace is formidable." The most valuable source were diaries written by Lionel Logue, played by Geoffrey Rush, the speech therapist who treated the king.

The film, which won the audience award at the Toronto film festival earlier this year, will be on general release in the UK in January.